The present invention relates to a wireless communication system, a transmitter and a receiver used therefor, and a wireless communication method.
In a related wireless communication system, a transmitter and a receiver perform wireless communication to each other using limited frequency bandwidth, and hence, available communication speed is also limited. If wireless communication is performed using a wide frequency bandwidth in order to increase the communication speed thereof, any interference may often occur with another wireless communication apparatus that uses various frequencies, thereby causing any deterioration in communication quality. Multiple input multiple output (MIMO) communication such that separate pieces of information is transmitted at a same frequency using multiple sets of antennas has been carried out to increase the communication speed (see Japanese Patent Application Publications Nos. 2003-338781 and 2004-254290).
FIG. 1 is a drawing for illustrating a wireless communication system that performs the MIMO communication. It is to be noted that two antennas are used at the transmitter side and the receiver side, respectively. A data-splitting unit 501 of a transmitter 50t splits transmission data stream DT to be transmitted into two sub-streams DTa, DTb and supplies them to a matrix-calculating unit 502. The matrix-calculating unit 502 receives the sub-streams DTa, DTb from the data-splitting unit 501 and multiplies the sub-streams DTa, DTb by matrices corresponding to any channel assumptions so as to be weighted according to the channel assumptions, thereby generating the transmission signals DUa, Dub. A signal-transmitting unit 503a is connected to an antenna 60at. The signal-transmitting unit 503a receives the transmission signal DUa and performs any modulation and frequency-conversion on the transmission signal DUa to generate an RF signal Sat and supply it to the antenna 60at. Similarly, a signal-transmitting unit 503b is connected to an antenna 60bt. The signal-transmitting unit 503b receives the transmission signal DUb and performs any modulation and frequency-conversion on the transmission signal DUb to generate an RF signal Sbt that has the same frequency as that of the RF signal Sat generated in the signal-transmitting unit 503a and supply it to the antenna 60bt. 
An antenna 60cr of a receiver 50r receives an RF signal Scr and supplies it to a signal-receiving unit 505c. An antenna 60dr of the receiver 50r receives an RF signal Sdr and supplies it to a signal-receiving unit 505d. The signal-receiving unit 505c performs any frequency-conversion and de-modulation on the RF signal Scr to generate a reception signal DQc and supply it to a matrix-calculating unit 506. The signal-receiving unit 505d performs any frequency-conversion and de-modulation on the RF signal Sdr to generate a reception signal DQd and supply it to the matrix-calculating unit 506.
The matrix-calculating unit 506 multiplies the reception signals DQc, DQd by matrices corresponding to any channel assumptions to extract a signal component transmitted from the antenna 60at and a signal component transmitted from the antenna 60bt, respectively, therefrom. The matrix-calculating unit 506 then supplies them to a data-reconfiguring unit 507 as sub-streams DRc, DRd. The data-reconfiguring unit 507 receives and reconfigures the sub-streams DRc, DRd to generate and output a reception data stream DR.
Thus, if the matrices are set corresponding to any channel assumptions, it is possible to extract only a signal transmitted from a desired antenna even when an antenna can receive signals transmitted from multiple antennas. If signals are transmitted from multiple antennas at the same frequency, it is possible to extract a signal transmitted from each of the antennas separately. When the extracted signals are reconfigured, original data can be given.
However, in the MIMO communication, the matrices are set corresponding to any channel assumptions and the multiplication by the matrices, the weighting, and an extraction of signal component have been performed. This causes a circuit to be complicated and expensive. Further, it is also preferable to establish any small correlation between transmission channels in order to extract a signal effectively. Thus, if a receiver is used in an outdoor channel where there is little reflected wave or an indoor channel in a wooded architecture, it is difficult to extract a signal effectively, in contrast with a case where the receiver is used in a multipath environment where there are many reflected waves. This may limit the communication speed. Alternatively, it is supposed that a reflected wave having a large amount of attenuation is used in the MIMO communication. Thus, it is difficult to expand a communication area with keeping a high communication speed.
In view of the above, it is desirable to provide a wireless communication system, a transmitter and a receiver used therefor, and a wireless communication method, which are simply configured and which enable the communication speed thereof to be easily increased.